This invention relates to a transferred electron effect device comprising a semiconductor body having an active region of n conductivity type formed of a semiconductor material having a relatively low mass, high mobility conduction band main minimum and at least one relatively high mass, low mobility conduction band satellite minimum and an injection zone adjoining the active region for causing electrons to be emitted, under the influence of an applied electric field, from the injection zone into the active region with an energy comparable to that of a relatively high mass, low mobility conduction band satellite minimum of the active region.
The transferred electron effect is discussed in detail in Chapter 11, pages 637 to 767 of the text book `Physics of Semiconductor Devices`, Second Edition, by S. M. Sze published in 1981 by John Wiley and Sons Inc. of New York. Briefly, in certain semiconductor materials such as gallium arsenide or indium phosphide the application of an electric field greater than a threshold or critical field enables transfer of electrons from the relatively low mass, high mobility conduction band main minimum (.GAMMA.) to a relatively high mass, low mobility conduction band satellite minimum ((L) for gallium arsenide or indium phosphide) so that the semiconductor material exhibits a bulk negative differential resistance, allowing charge instabilities to grow to form accumulation or dipole layers. Semiconductor materials exhibiting such a bulk negative differential resistance can, as first observed by Gunn and as discussed in the aforementioned Chapter of the text book by Sze, be used to form devices which, when a dc electric field greater than the critical field is applied, generate a coherent microwave output.
As will be appreciated from the above, in order to enable electrons to be transferred to a relatively high mass, high energy, low mobility satellite minimum (L) to obtain the negative differential resistance characteristic, sufficient energy must be imparted to the electrons by the applied electric field. Conventionally, a transferred electron effect device comprises a relatively lowly doped n-conductivity type active region, for example an active region with a dopant concentration of about 1.times.10.sup.16 atoms cm.sup.-3, of an appropriate semiconductor material, for example gallium arsenide or indium phosphide, with relatively highly doped n-conductivity type regions being provided at opposed surfaces of the active region to enable ohmic contact to cathode and anode electrodes across which the electric field is to be applied. With such a conventional ohmic contact structure, electrons accelerated by the electric field do not achieve sufficient energy to transfer to the satellite minima until they have traversed a given distance along the semiconductor body between the cathode and anode contacts. Thus, in such a conventional ohmic contact structure, the injection zone comprises a part of the active region and forms an acceleration zone in which the electrons in the main conduction band minima are heated. Accordingly, the accumulation or dipole layers which result in the microwave oscillation grow some distance from the cathode and there is in effect a dead zone within the device. For a given applied electric field, the length of the acceleration zone is effectively fixed while the frequency of the microwave output is inversely proportional to the length of the device. Accordingly, as demand occurs for devices capable of providing higher and higher frequency microwave outputs, the proportion of the length of the device taken up by the acceleration zone or dead zone increases, adversely affecting device performance and efficiency.
Various methods have been proposed for facilitating the acceleration of electrons into the relatively high mass, low mobility high energy satellite minima (L). In particular, it has been proposed that the field at the cathode may be increased by providing a notch in the doping profile adjacent the cathode as described in, for example, an article entitled `Monte Carlo simulation of a Millimeter-wave Gunn-effect Relaxation Oscillator` by John W. Tully published in the IEEE Transactions on Electronic Devices Volume ED-30 No. 6, June 1983 at pages 566 to 571.
However, although the use of a cathode notch may reduce the length of the dead zone, the dead zone is not eliminated and the device still relies on the same basic essentially diffusive heating mechanism to impart the necessary energy to the electrons. Furthermore, the potential drop across the relatively lowly doped cathode notch is large so that although the length of the dead zone may be reduced, the field is increased. As will be appreciated, the power losses in a device are equal to the product IV of the current and applied voltage. In order to reduce power losses for a given current I, the voltage V must be reduced. Although the necessary applied voltage is reduced by using a cathode notch, the advantage gained in practice is less than might be expected from the reduction in the length of the dead zone.
Another proposal has been to replace the ohmic cathode contact with a reverse-biased Schottky contact as described in, for example, an article entitled `Injection Properties of Contacts to InP` by H. Rees published in the Institute of Physics Conference Series, No. 22, at pages 105 to 115 (1974). However, for such Schottky barrier injection zones to be effective, it is necessary for Schottky barriers to be prepared which have a barrier height considerably lower than the normal Schottky barrier height which is pinned by surface states and shows only a weak dependence on the work function of the metallic cathode contact or electrode. Unfortunately, the preparation of such Schottky barriers either by contact processing or by use of doped surface layers has proved unreliable, particularly where the semiconductor body comprises a gallium arsenide body.